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Implement prefetching via posix_fadvise() for bitmap index scans. A new

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GUC variable effective_io_concurrency controls how many concurrent block
prefetch requests will be issued.

(The best way to handle this for plain index scans is still under debate,
so that part is not applied yet --- tgl)

Greg Stark
This commit is contained in:
Tom Lane
2009-01-12 05:10:45 +00:00
parent 1a37056a74
commit b7b8f0b609
15 changed files with 422 additions and 17 deletions
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51
doc/src/sgml/config.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.204 2009/01/09 10:13:18 mha Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.205 2009/01/12 05:10:44 tgl Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
@ -1203,6 +1203,55 @@ SET ENABLE_SEQSCAN TO OFF;
queries.
</para>
</sect2>
<sect2 id="runtime-config-resource-async-behavior">
<title>Asynchronous Behavior</title>
<variablelist>
<varlistentry id="guc-effective-io-concurrency" xreflabel="effective_io_concurrency">
<term><varname>effective_io_concurrency</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>effective_io_concurrency</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the number of concurrent disk I/O operations that
<productname>PostgreSQL</> expects can be executed
simultaneously. Raising this value will increase the number of I/O
operations that any individual <productname>PostgreSQL</> session
attempts to initiate in parallel. The allowed range is 1 to 1000,
or zero to disable issuance of asynchronous I/O requests.
</para>
<para>
A good starting point for this setting is the number of separate
drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
database. (For RAID 5 the parity drive should not be counted.)
However, if the database is often busy with multiple queries issued in
concurrent sessions, lower values may be sufficient to keep the disk
array busy. A value higher than needed to keep the disks busy will
only result in extra CPU overhead.
</para>
<para>
For more exotic systems, such as memory-based storage or a RAID array
that is limited by bus bandwidth, the correct value might be the
number of I/O paths available. Some experimentation may be needed
to find the best value.
</para>
<para>
Asynchronous I/O depends on an effective <function>posix_fadvise</>
function, which some operating systems lack. If the function is not
present then setting this parameter to anything but zero will result
in an error. On some operating systems the function is present but
does not actually do anything. On such systems setting a nonzero
value will add CPU overhead without improving performance.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-wal">